As recent technology continues to rapidly increase performance, operation and compactness levels of electronic devices, there is an increasing demand to further reduce sizes and weights of electronic parts for use in such electronic devices. Accordingly, there also is a strong demand to further improve properties of electronic part materials, including heat resistance, mechanical strength and electrical properties.
A multilayer printed board for use in electronic devices has plural insulating layers arranged alternately with circuit patterns. Conventionally used for such an insulating layer is a hardened product of a thermosetting resin prepreg made by impregnating a thermosetting resin in a glass cloth, and a resin sheet comprised of a thermosetting or photosetting resin, for example.
It is to be desired that an interlayer spacing of the insulating layers in the multilayer printed board is narrowed considerably to increase a density and a thickness of the multilayer printed board. This raises a need for insulating layers which incorporate a thin glass cloth or consist of a resin containing an inorganic filler. The use of such a thin glass cloth or inorganic filler not only ensures a mechanical strength or other mechanical properties of the multilayer printed board even if having considerably narrow interlayer spacings, but also reduces a dimensional change of the multilayer printed board prior to and subsequent to application of a thermal history, i.e., lowers its linear expansion coefficient (thermal linear expansion coefficient).
Thermosetting resin materials incorporating a large amount of inorganic fillers are conventionally known to constitute such resins containing inorganic fillers that exhibit the above-described functions.
The below-specified Patent Literature 1 discloses a method for fabricating a multilayer printed circuit board (multilayer printed board) by using an epoxy adhesive film as an insulating layer. In the fabrication method described in Patent Literature 1, for example, a bifunctional epoxy resin and a bifunctional phenol in a solvent are polymerized under the presence of a catalyst to obtain a high-molecular weight epoxy polymer having a weight average molecular weight of at least 50,000. Nonfibrous inorganic fillers having a mean particle size of 0.8-5 μm are incorporated in a varnish comprised mainly of the above epoxy polymer, a polyfunctional epoxy resin, a hardener and a crosslinker. This varnish is coated onto one or both surfaces of a support. Subsequent removal of the solvent results in obtaining a multilayer printed circuit board. Patent Literature 1: Japanese Patent Laying-Open No. 2000-183539